In a large cast iron skillet or braiser, heat olive oil and butter over medium heat until melted and frothy. Add the onions and shallots and sprinkle liberally with salt. Saute until golden brown.

Pour the beef broth, red wine and Worcestershire sauce over the onion mixture. Bring the liquid up to a boil, then reduce to a simmer. Add the thyme and bay leaf and cover. Let the onions braise over low heat for 40-50 minutes or until most of the liquid is absorbed. Remove the onions from the heat and let cool completely.

When the onions have reached room temperature, roll the puff pastry out until it's an 1/8 of an inch thick.

Cut circles out of the pastry using a 3-inch biscuit cutter. Transfer the circles to a baking sheet.

Top each pastry circle with a generous amount of the onion mixture. Make sure to leave a clean 1/2-inch border around the onions. Place a piece or two of Gruyere on top of the onion mixture and transfer the tarts to the fridge to chill for 1 hour.

While the tarts are chilling, preheat the oven to 400F.

Bake the tarts for 20 minutes or until golden brown and bubbling.

Serve tarts warm or at room temperature.